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But if you’re overweight, black, older or live in the Northeast, there’s a good chance you’re not getting enough vitamin D in your diet, said study co-author Cedric F. Garland, a professor of medicine at the University of California, San Diego.

And that could put you at risk, he added.

Garland and his colleagues examined 63 previous studies that looked at possible links between several types of cancer and vitamin D deficiency. Their study appears in the current online edition of the American Journal of Public Health, and will appear in the February 2006 print edition.

According to the researchers, the studies — from 1966 to 2004 — suggest that vitamin D can reduce the risk of colon, breast and ovarian cancers, among others, by as much as 50 percent.

However, the debate over the value of vitamin D isn’t over, said Lona Sandon, a spokeswoman for the American Dietetic Association.

The new research suggests a link between too little vitamin D and cancer, but doesn’t confirm it, she said.

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“Vitamin D’s main role is to keep the balance of calcium and phosphorous in the blood, which helps keep bones strong,” Sandon said. “However, a lesser-known role is how it regulates cell growth and determines what a cell becomes. A vitamin D deficiency may allow cells to become cancerous rather than becoming healthy cells.”

The study authors found that several groups of people had low levels of vitamin D. Residents of the Northeast made up one group, perhaps because they miss out on vitamin D that’s absorbed during exposure to the sun, Garland said. The obese had low levels, too, perhaps because they have trouble metabolizing vitamin D through their fatty tissues.

Other groups with low vitamin D levels include blacks — they’re five times more likely to be deficient than whites — and the elderly, the researchers found.

“As we age, we lose the ability to convert vitamin D into its usable form, so elderly people are at greater risk,” Sandon said.

And the increased skin pigmentation of blacks reduces their ability to synthesize vitamin D, the researchers said.

So what to do? The experts are divided on that answer.

Garland urges everyone to consume 1,000 International Units (IUs) a day of the active form of Vitamin D — also known by its human form, Vitamin D3 — which comes in yogurt, cheese, orange juice, fatty fish and milk.

By contrast, Sandon said adults aged 19 to 50 should get 200 IUs a day, equivalent to two glasses of fortified milk. People aged 50 to 70 should get 400 IUs, she said, while those 71 and older should get 700. But she acknowledged that “it is difficult to get this much vitamin D from food alone.

She recommends that people take brief walks during lunch to get exposure to vitamin D from the sun.

But what about seniors or those who refuse to change their diets or their habits? “A supplement of the active form of vitamin D would be the next option for those who just will not make even small changes, and likely to be a must in people over 50,” Sandon said.